The present invention relates to a bulk acoustic wave (BAW) device and its manufacture and particularly a filter comprising solidly mounted BAW resonators. The invention also relates to communications equipment (for example a radio frequency receiver and/or transmitter) comprising such filters.
Mobile communications products, such as cellular phone handsets, are required to be small and light. It is predicted that in the future even smaller communication devices will be available integrated into, for example, wrist-watches and clothing. All such products require radio-frequency (RF) filters approximately covering the range 0.5 GHz to 10 GHz to protect the received signal from interference, either from the transmitter in the same handset and/or from unwanted externally generated signals. These filters must have low pass-band insertion loss (typically less than 2 Db) in order to achieve adequate signal-to-noise ratio. To achieve this, the resonators, which are the basic building blocks of filters, must have high quality factor Q. This is defined as the energy stored per cycle in a resonator at the resonant frequency divided by the energy lost per cycle by the resonator at the same frequency. Typically, values for Q in excess of 500 are desirable and achievable.
Resonators normally employ some form of standing wave in a cavity. Both discrete and distributed reflections may be used. Conventional bulk acoustic wave and surface acoustic wave (SAW) resonators are examples of resonators relying on these two options. To keep size to a minimum, discrete reflections are preferred because the length of the cavity is then typically only xc2xd a wavelength of the mode employed at the resonant frequency. Thus BAW resonators are potentially much smaller than SAW resonators for which a cavity length of the order of 100 wavelengths may be required, and are preferred for this reason.
Resonators are available that rely on acoustic waves or electromagnetic waves. Acoustic wave resonators are preferred to those employing electromagnetic waves for two principal reasons. Firstly, the velocity of acoustic waves propagating in a material is typically 4 to 5 orders of magnitude lower than the velocity of electromagnetic waves, so that a substantial size reduction is possible for any given frequency. Secondly, achievable mechanical quality factors are typically larger than achievable electrical quality factors for the same materials.
Two general types of BAW resonators have been studied for RF applications. In the first of these a thin membrane forms the resonating cavity. This approach is unattractive because the membranes are fragile and subject to buckling caused by stress. In the second, so-called SMRs (solidly-mounted resonators) are used as shown in FIG. 1. In devices such as these, one or more acoustically mismatched layers 2 are mounted on a substrate 4 and act to reflect the acoustic wave. Upper 6 and lower 8 electrodes are formed on the substrate 4 separated by a piezoelectric layer 10. Since the reflector layer(s) are deposited on a solid substrate, the structure of a SMR is robust.
In the BAW resonator shown in FIG. 1, the required conversion between electrical and mechanical energy is achieved by the layer 10 of piezoelectric material arranged between two metal layers in which electrodes 61, 62 and 8 are formed. Although the SMR employs a more distributed reflection than the thin membrane resonator, resonator size is not significantly increased because thickness is predominantly determined by the substrate in both cases. Each upper electrode 61 and 62 defines an individual resonator with the underlying piezoelectric layer and lower electrode. These two resonators are effectively electrically connected in series, with the common lower electrode 8 at the junction between them. A resonator is a one-port device. In the construction shown in FIG. 1, its two terminals are formed by electrodes 61 and 62.
RF filters reported to date have been constructed by electrically connecting SMRs in either a ladder or lattice configuration. Ladder configurations of the filters have demonstrated good performance with passband insertion loss at less than 2 dB and very low levels of spurious response. However, there are a number of disadvantages with such arrangements. For example, at frequencies removed from the acoustic resonances, each resonator appears as a capacitor, so the overall filter stop-band response is essentially that of a capacitor network. This leads to a requirement for additional resonators just to reduce the stop-band. Consequently, both the area occupied and the insertion loss in the pass-band are increased without improving selectivity. A large number of resonators is required for even a moderate stop-band level (e.g. a minimum of 9 resonators for approximately 45 dB stop-band). With the drive towards the miniaturization of filters in RF applications this is a serious problem.
In addition, series and shunt resonators in the ladder configuration are required to be centred on different frequencies due to the arrangement of the individual resonators. This means, for example, that an additional mass-loading layer, of very precise thickness, must be deposited on the shunt resonators to reduce their anti-resonance (minimum-admittance) frequency to the same as the resonance (minimum-impedance) frequency of the series resonators.
FIG. 2 shows a schematic representation of an electrical equivalent circuit model for a conventional BAW resonator such as that shown in FIG. 1. C0 is the static capacitance of the resonator, Cm and Lm are respectively the motional capacitance and inductance, and Rmis the motional resistance which characterises the mechanical losses of the resonator. The resonant frequency is given by f0=1/[2xcfx80(CmLm)], and the unloaded quality factor is given by Qu=(2xcfx80f0Lm)/Rm.
The manufacturing process for a thin film bulk acoustic wave resonator will be known by those skilled in the art. For example, International Patent Application number WO98/16957 discloses a thin film bulk acoustic wave resonator and a method of manufacturing the same, the contents of which are incorporated herein by reference.
As explained above, in a filter it is usual to arrange more than one resonator in a lattice or ladder configuration connected electrically to each other to obtain optimum filter characteristics. Connecting a number of filters like the one shown in FIG. 1 causes an inherent lack of design flexibility due to the presence of the static capacitance C0 in each resonator. As a consequence, approximations to standard ideal filter types such as Butterworth or Chebyshev are not readily implemented. The electrical connection of the resonators in a ladder configuration also produces the need for the series and shunt resonators to be centred on different frequencies.
According to the present invention, there is provided a bulk acoustic wave device comprising:
one or more acoustic reflector layers formed on a substrate;
a lower electrode formed on said acoustic reflector layer or layers;
a piezoelectric layer formed on said lower electrode; and,
at least three upper electrodes formed over said piezoelectric layer each upper electrode at least partially overlying the lower electrode and defining with an underlying piezoelectric layer portion and the lower electrode a resonator element, in which said upper electrodes are laterally spaced such that a signal applied between one of said upper electrodes and said lower electrode at a resonant frequency of the device is coupled to the other resonator elements by acoustic coupling between the piezoelectric layer portions and in which the upper electrodes are arranged so that there are two outer and at least one inner upper electrodes and in which the or each inner upper electrode is electrically connected to the lower electrode.
The invention provides a device employing SMRs, which are acoustically rather than electrically coupled. This acoustic coupling enables the device to be smaller and leads to more flexible filter design. The electrical connection between electrodes is such as to provide a fixed potential between the inner upper electrode(s) and the lower electrode. This potential is preferably zero, and they are preferably earthed. This earthing provides electromagnetic shielding between the input and output of the device, by reducing the parasitic capacitance between the input and output upper electrodes.
The connection between the inner upper electrode and the lower electrode is typically by means of one or more vias.
Preferably, each resonator is centred on the same resonant frequency. This enables a simpler layer structure and is possible because all the resonators form shunt arms, the equivalent of series arms being implicitly provided by the acoustic coupling between adjacent resonators. Thus, it is possible to increase the number of resonators in the device to achieve a higher-order filter, rather than just to decrease the stop-band level as in existing designs. It is thus possible to implement a filter design without resorting to the conventional ladder configuration in which shunt resonators have an anti-resonance frequency set at the same frequency as the resonance frequency of series resonators. The invention improves the simplicity of manufacture and therefore reduces the costs of manufacturing the device.
Preferably, the separation between the laterally spaced upper electrodes is between 0.5 and 2.0 xcexcm and more preferably between 0.7 and 1.3 xcexcm. Having a separation of the order of 1 xcexcm ensures that the acoustic coupling between adjacent upper electrodes and the associated resonators is achieved.
Preferably, the piezoelectric layer is selected to have a thickness substantially equal to xc2xd a wavelength of the dominant acoustic mode at the resonant frequency.
According to a second aspect of the present invention, there is provided a method of designing a bulk acoustic wave device having two outer resonators and at least one inner resonator each of the resonators being laterally spaced with respect to the adjacent resonator or resonators and each resonator having an upper electrode, a lower electrode and an interposed piezoelectric layer, the method comprising the steps of:
determining an optimum upper electrode width for energy-trapping a single mode of oscillation within the locality of the electrode;
determining normalized low-pass prototype values for a selected filter type and from these determining the loaded quality factors of the outer resonators;
determining required areas for the outer upper electrodes of the filter in dependence on loaded quality factors of the outer resonators;
in dependence on these areas, and the optimum electrode width for energy-trapping a single mode, calculating the lengths of each of the outer electrodes;
from the normalized low-pass prototype values determining the interresonator coupling coefficients, and then determining the widths of the gaps between the resonators, in dependence on a previously-determined relationship between coupling coefficient and inter-resonator spacing.